Conventionally, there has been known a method of alternately supplying a raw material gas and a reactive gas to a substrate such as, for example, a semiconductor wafer, thereby depositing a thin film of a reaction product generated by a reaction between the raw material gas and the reactive gas on the substrate.
There has also been known a method of generating a mixed gas by mixing a raw material gas and a purge gas in a buffer tank, supplying the mixed gas and a reactive gas into a processing container, thereby depositing a thin film on a substrate (see, e.g., Japanese Patent Laid-Open Publication No. 2014-007378). In this method, the in-plane uniformity of a film thickness is improved in a state where the film forming rate is kept high.